1. Field of the Invention
The present invention relates to a method for producing a foamed article by which a foamed article having a high expansion ratio, having uniform fine cells in a high closed cell ratio and capable of being recycled, which is utilizable as heat insulating materials for construction, cushioning materials for wrapping, cushioning floats for ships, floats for sports and leisure and the like, can be obtained readily and safely using a clean inorganic gas as a foaming agent, and to a foamed article obtained by the method.
2. Description of the Related Art
Hitherto, as a method for producing a foamed thermoplastic resin article, there have been known a chemical foaming method in which a heat-decomposable foaming agent is mixed with a thermoplastic resin by kneading, and foaming is performed by heating over the decomposition temperature of the foaming agent, and a gas foaming method in which an organic gas having a boiling point not higher than the softening point of a resin, such as butane, pentane and dichlorodifluoromethane (Freon R-12), or a volatile liquid is supplied into the molten resin under pressure, and then the mixture is discharged to a lower pressure to be foamed.
Among the above-mentioned conventional foaming methods, in the case of using a chemical foaming agent, a foamed article having uniform fine closed cells can be obtained. The foamed article, however, contains decomposition residues of the foaming agent to raise problems of discoloring, generation of smell, deterioration in physical properties, etc.
On the other hand, in the case of the gas foaming method, when a foaming agent is a low boiling organic solvent such as butane and pentane, there is a risk of fire and explosion in the manufacturing process. When the foaming agent is chlorofluorocarbon compounds, there can readily be obtained a foamed article with a high expansion ratio having uniform fine closed cells without any risk of explosion, but there are environmental problems such as destroying the ozone layer. Thus the use of such chlorofluorocarbon compounds is proceeding toward a complete removal.
As methods for solving the foregoing problems, for example, a method in which an inorganic gas, such as carbon dioxide, nitrogen and the air, is mixed with a polypropylene resin, and the mixture is then extruded to foam, as described in Japanese Laid-open Patent Publication No. 60 (1985)-31538.
However, in the above-mentioned method, especially when intending to obtain a foamed article by discharging from a circular die having a tubular channel, an undesirable apparent defect of many streaks was often observed. When an organic solvent is used, such an apparent defect is not observed, or is slightly observed, and it does not damage commercial values of products. On the contrary, the apparent defect, which seems to occur inherently when the inorganic gas is used, seriously damages the commercial values of the products.
The present invention intends to provide a method by which a foamed thermoplastic resin article not having any apparent defect observed in the aforementioned prior art can be obtained using a inorganic carbon dioxide gas as a foaming agent.
The present inventors have intensively studied to improve the above-mentioned streak-like defect by a production method. As a result, their finding that a foamed article with good appearance can be obtained by taking the following action has accomplished the present invention.
That is, the present invention provides;
1. A method for producing a foamed resin article comprising a melting step of feeding a thermoplastic resin into an extruder and heating it to melt, a kneading step of mixing an inorganic gas with the thermoplastic resin in the extruder to produce a kneaded material, and a foaming step of discharging the kneaded material from a mold having a tubular channel mounted to the end of the extruder to form the foamed article,
wherein extrusion molding is carried out within the range where a numeric X, which is defined by the following expression:
X=100P+3.5H
xe2x80x83where a resin feeding pressure of the thermoplastic resin from the extruder to the mold is represented by H (kgf/cm2) and an expansion ratio is represented by P, and the resin feeding pressure H satisfy the following expressions simultaneously:
780xe2x89xa6Xxe2x89xa64000,
and
80xe2x89xa6Hxe2x89xa61000
2. The method for producing a foamed resin article according to above 1, wherein a numeric Y, which is defined by the following expression:
Y=100P+3.5(Hxe2x88x9210G)
where the amount of the inorganic gas supplied to 100 g of the thermoplastic resin is represented by G g, satisfies the following expression:
760xe2x89xa6Yxe2x89xa64000
3. The method for producing a foamed resin article according to above 1 or 2, wherein the shape of the tip (lip) of the mold having a tubular channel satisfies the following expressions simultaneously:
0.1xe2x89xa6Lxe2x89xa61;
0.1xe2x89xa6tan xcex8xe2x89xa66;
and
5xe2x89xa6Txe2x89xa6500
where a lip gap, a taper angle, and a taper land are represented by L (mm), xcex8, and T (mm), respectively.
4. The method of producing a foamed resin article according to any one of above 1 to 3, wherein, as for the shape of the tip (lip) of the mold having a tubular channel, an angle xcex1 which is formed by the center axis of the core of the lip and the taper land satisfies the following expression:
0.1xe2x89xa6tan xcex1xe2x89xa66
5. The method for producing a foamed resin article according to any one of above 1 to 4, wherein the inorganic gas is carbon dioxide.
6. The method for producing a foamed resin article according to any one of above 1 to 5, wherein the thermoplastic resin is a propylene-based polymer.
7. The method for producing a foamed resin article according to above 6, wherein the propylene-based polymer is a polymer which is obtained by continuous production comprising polymerizing, in the first stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (A) having a limiting viscosity of 5 dl/g or more and successively polymerizing, in the second stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (B) having a limiting viscosity of less than 3 dl/g and which is composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B) wherein a content of the polymer (A) is in the range of 0.05% by weight or more and less than 35% by weight, a limiting viscosity of the whole polymer is less than 3 dl/g, and Mw/Mn is less than 10.
8. The method for producing a foamed resin article according to claim 7, wherein the propylene-based polymer satisfies the following expression:
WAxe2x89xa7400xc3x97EXP(xe2x88x920.6[xcex7A])
where [xcex7A] (dl/g) is the limiting viscosity of the crystalline propylene-based polymer (A) and WA (% by weight) is the crystalline propylene-based polymer (A) content in the whole polymer.
9. The method for producing a foamed resin article according to above 7 or 8, wherein, the propylene-based polymer is composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B), each of them being a propylene homopolymer, a random copolymer of propylene and 10% by weight or less of ethylene, a random copolymer of propylene and 30% by weight or less of butene, or a random terpolymer of propylene, 10% by weight or less of ethylene and 30% by weight or less of butene.
10. The method for producing a foamed resin article according to any one of above 7 to 9, wherein, as for the propylene-based polymer, the crystalline propylene-based polymer (A) has a limiting viscosity [xcex7A] of 7 dl/g or more.
11. The method for producing a foamed resin article according to any one of above 7 to 10, wherein, as for the propylene-based polymer, the crystalline propylene-based polymer (A) contains ethylene, as a comonomer, in a proportion of 1% by weight or more and 10% by weight or less.
12. The method for producing a foamed resin article according to any one of above 7 to 11, wherein the propylene-based polymer is one produced continuously by a production method wherein a catalyst containing Ti, Mg and halogen as essential components is used, the polymerization rate of the crystalline propylene-based polymer (A) in the first step is 2000 g or more per gram of the catalyst per hour and the polymerization rate of the crystalline propylene-based polymer (B) in the second step is 4000 g or more per gram of the catalyst per hour.
13. A foamed resin article which is obtained by the method of any one of above 1 to 12.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, and variations such as xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integer or step.
The resin feeding pressure (H; kgf/cm2) and the expansion ratio (P) are defined and measured as described below.
Resin feeding pressure (H): a resin pressure measured by a pressure gauge mounted to the connecting portion (head) from the end of the extruder to the mold (circular die).
Expansion ratio (P): a ratio obtained by dividing the specific gravity of the raw material (unfoamed material) by the specific gravity of the foamed article determined by a conventional specific gravimeter.
In the case of X less than 780, the expansion ratio becomes small or a defect of many-steak-like uneven pattern appears in the surface of the foamed article. In the case of X  greater than 4000, the apparatus unfavorably has too much load (pressure and torque).
In the case of H less than 80, a foamed article with a high expansion ratio can not be obtained. In the case of H greater than 1000, the apparatus also has unfavorably much load.
The expansion ratio preferably ranges from 2 to 50. When it is less than 2, the product has characteristics close to those of the resin itself rather than those of a foamed article. When it is more than 50, strength of the foamed article becomes poor.
In the aforementioned method for producing a foamed thermoplastic resin article, it is preferable that a numeric Y, which is defined by the following expression:
Y=100P+3.5(Hxe2x88x9210G)
where the amount of the inorganic gas supplied to 100 g of the thermoplastic resin is represented by G g, satisfies the following expression:
760xe2x89xa6Yxe2x89xa64000
In the case of Y less than 760, the expansion ratio becomes small or the defect of many-streak-like uneven pattern appears in the surface of the foamed article. In the case of Y greater than 4000, the apparatus unfavorably has too much load (pressure and torque).
In the method of the present invention for producing a foamed thermoplastic resin article, the shape of the tip (lip) of the mold having a tubular channel mounted to the end of the extruder preferably satisfies the following expressions simultaneously:
0.1xe2x89xa6Lxe2x89xa61;
0.1xe2x89xa6tan xcex8xe2x89xa66;
and
5xe2x89xa6Txe2x89xa6500
where a lip gap, a taper angle, and a taper land are represented by L (mm), xcex8, and T (mm), respectively.
L, xcex8 and T are defined as follows.
As for the mold having a tubular channel, its tip portion is called a lip, which is composed of a cylindrical mold called an inner lip and a pipe-like mold called an outer lip, the inner lip and the outer lip defining the tubular channel therebetween. The lip gap L is the gap lying between the inner lip and the outer lip at the tip of the lip. In the mold having a tubular channel, the channel lying between the position where the gap between the inner lip and the outer lip becomes more than 3 mm and the tip of the lip is defined as a land. In the land, the portion where the surfaces of the inner and outer lips facing the channel form an angle of 4 degrees or more is defined as the taper land T. When, for example, the channel has a curved surface, although the aforementioned angle varies depending upon positions in the channel, such a case is encompassed in the present invention and the maximum value of the angle is defined as xcex8.
In the case of L less than 0.1, cells in the foamed article are undesirably liable to break. The cause of this is presumed to be an excessively high shear rate at the lip or an abnormal temperature rise. The case of L greater than 1 is undesirable on the ground that it is difficult to keep high the resin feeding pressure H and it is difficult to achieve the requirements on the ranges of X and H. The more preferable range is 0.2xe2x89xa6Lxe2x89xa60.5.
In the case of tan xcex8xe2x89xa60.1, the defect of the streak-like unevenness often occurs. The case of tan xcex8 greater than 6 is undesirable on the ground that the resin is liable to stay in the taper land portion and the product tends to have portions with uneven expansion ratios. The more preferable range is 0.15xe2x89xa6tan xcex8xe2x89xa62.
In the range of Txe2x89xa65, the defect of the streak-like unevenness often occurs. The case of T greater than 500 is undesirable in view of a facility cost because the apparatus becomes large. The more preferable range is 10xe2x89xa6Txe2x89xa650.
Furthermore, the present invention preferably uses a mold having a tubular channel mounted to the end of the extruder in which the shape of the tip (lip) of the mold satisfies the following expression:
0.1xe2x89xa6tan xcex1xe2x89xa66
where an angle formed by the center axis of the core of the lip and the inner lip defining the taper land is represented by xcex1.
In the case of tan xcex1xe2x89xa60.1, the defect of streak-like unevenness often occurs. The case of tan xcex1 greater than 6 is undesirable on the ground that the resin is liable to stay in the taper land portion and the product tends to have portions where the expansion ratio is uneven. The more preferable range is 0.5xe2x89xa6tan xcex1xe2x89xa62. When, for example, the channel has a curved surface, although the aforementioned angle varies depending upon positions in the channel, such a case is encompassed in the present invention and the maximum value of the angle is defined as xcex1.
The more preferable conditions of the lip are that the taper land is located near the tip of the lip and that the distance M (mm) between the taper land and the tip of the lip satisfies the expression of 0xe2x89xa6Mxe2x89xa650, preferably 1xe2x89xa6Mxe2x89xa620. When M is more than that value, an undesirable apparent defect of many streaks was often observed.
The inorganic gas to be used in the present invention, which is a foaming agent, is a material which is in a gaseous state at ordinary temperature and ordinary pressure. Particularly preferred is carbon dioxide.
The thermoplastic resin to be used in the present invention as a material from which the foamed article is formed is preferably a propylene-based polymer.
The aforementioned propylene-based polymer is preferably a polymer which is obtained by continuous production comprising polymerizing, in the first stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (A) having a limiting viscosity of 5 dl/g or more (hereinafter, sometimes referred to as xe2x80x9cpolymer (A)xe2x80x9d) and successively polymerizing, in the second stage, monomers mainly composed of propylene to produce a crystalline propylene-based polymer (B) having a limiting viscosity of less than 3 dl/g (hereinafter, sometimes referred to as xe2x80x9cpolymer (B)xe2x80x9d) and which is composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B) wherein a content of the polymer (A) is in the range of 0.05% by weight or more and less than 35% by weight, a limiting viscosity of the whole polymer is less than 3 dl/g, and Mw/Mn is less than 10. (Such a propylene-based polymer is sometimes abbreviated to PP-LG.)
The xe2x80x9cpolymer which is obtained by a continuous productionxe2x80x9d includes polymers obtained by a batch polymerization method in which, in the same polymerization vessel, the crystalline propylene-based polymer (A) is produced by polymerization, followed by production of the crystalline propylene-based polymer (B) by polymerization, a polymerization method in which the polymerization vessels consisting of at least two vessels are connected in series, and after production of the polymer (A), the product is transferred to the next polymerization vessel, followed by production of the polymer (B) in the polymerization vessel, or the like.
It is particularly preferable that the crystalline propylene-based polymer (B), which is a component of the PP-LG, is a propylene-based polymer obtained by production following the production of the crystalline propylene-based polymer (A). Some mere blends of a crystalline propylene-based polymer having a limiting viscosity of 5 dl/g or more and a propylene-based polymer having a limiting viscosity of less than 3 dl/g can not improve or insufficiently improve their melt strength.
The crystalline propylene-base polymer (A) preferably has a limiting viscosity of 5 dl/g or more. When the limiting viscosity of the polymer (A) is less than 5 dl/g, the propylene-based polymer is inferior in melt strength, and hence the object of the present invention is difficult to be achieved. The limiting viscosity of the polymer is more preferably 6 dl/g or more, still more preferably 7 dl/g or more.
In the continuous polymerization, the limiting viscosity of the polymer (B) can be set in the above range by suitably choosing production conditions of the polymer (B). The limiting viscosity of the polymer (B) can usually be calculated from the limiting viscosities of the final polymer and the polymer (A) under the assumption that additivity of limiting viscosities is established.
The proportion of the crystalline propylene-based polymer (A) in the whole propylene-based polymer is preferably 0.05% by weight or more and less than 35% by weight. When the proportion is less than 0.05% by weight, the melt strength becomes poor. When the proportion is 35% by weight or more, the elongation characteristics become poor. The proportion is more preferably 0.3% by weight or more and 20% by weight or less. The less proportion of the crystalline propylene-based polymer (A) is desirable as long as the polymer (A) satisfies the melt strength requirement. It is also a preferred embodiment for obtaining a propylene-based polymer having a polymer (A) content of 0.3% by weight or more and 20% by weight or less in which a propylene-based polymer having a polymer (A) content of 20% by weight or more and less than 35% by weight is produced first and the proportion of the polymer (A) is adjusted by adding an ingredient corresponding to the polymer (B) in the melting step or the kneading step.
The limiting viscosity of the crystalline propylene-based polymer (B) is preferably less than 3 dl/g. When it is 3 dl/g or more, the limiting viscosity of the whole polymer becomes so high that the polymer may become inferior in fluidity and a problem in workability may be caused. Even if the viscosity of the whole material is adjusted by addition of other ingredients, there occur problems in miscibility and the like.
The limiting viscosity of the whole PP-LG is preferably less than 3 dl/g. When the limiting viscosity is 3 dl/g or more, the fluidity of the whole material may be poor and a problem in workability may occur. The limiting viscosity of the whole PP-LG is more preferably 1 dl/g or more and less than 3 dl/g.
The PP-LG preferably has a ratio of a weight mean molecular weight (Mw) to a number mean molecular weight (Mn) of less than 10. When Mw/Mn is 10 or more, the appearance of the resulting article is sometimes poor, or the elongation characteristics sometimes deteriorate.
From the viewpoint of melt strength, the propylene-based polymer to be used in the present invention preferably satisfies the following expression:
WAxe2x89xa7400xc3x97EXP(xe2x88x920.6[xcex7A])
where [xcex7A] (dl/g) is the limiting viscosity of the crystalline propylene-based polymer (A) and WA (% by weight) is the crystalline propylene-based polymer (A) content in the whole polymer. When WA is less than that value, the melt strength is insufficiently improved.
In the present invention, the propylene-based polymer is preferably composed of the crystalline propylene-based polymer (A) and the crystalline propylene-based polymer (B), each of them being a propylene homopolymer, a random copolymer of propylene and 10% by weight or less of ethylene, a random copolymer of propylene and 30% by weight or less of butene, or a random terpolymer of propylene, 10% by weight or less of ethylene and 30% by weight or less of butene. The polymers (A) and (B) may have the same composition.
Especially, the propylene-based polymer preferably has a crystalline propylene-based polymer (A) containing ethylene, as a comonomer, in a proportion of 1% by weight or more and 10% by weight or less.
A polymer which is continuously produced by a production method wherein a catalyst containing Ti, Mg and halogen as essential components is used, the polymerization rate of the crystalline propylene-based polymer (A) in the first stage is 2000 g or more per gram of the catalyst per hour and the polymerization rate of the crystalline propylene-based polymer (B) in the second stage is 4000 g (being twice as much as that in first stage) or more per gram of the catalyst per hour, is particularly preferred as the propylene-based polymer. It is noted that xe2x80x9cper gram of the catalystxe2x80x9d herein means one gram of a solid catalyst containing Ti, Mg and halogen as essential components.
Use of the catalyst system and the production method resulting in that the polymerization rate in the polymerization of the crystalline propylene-based polymer (A) is less than 2000 g per gram of the catalyst per hour sometimes results in a decrease in production efficiency, coloring of the polymer due to catalyst residues, a reduction of heat resistance and the like.
The polymerization temperature of the crystalline propylene-based polymer (A) ranges from 20xc2x0 C. to 150xc2x0 C., preferably from 35xc2x0 C. to 95xc2x0 C.
The polymerization rate of the crystalline propylene-based polymer (B) is preferably adjusted by selecting polymerization conditions so as to be twice or more, more preferably three times or more, as much as the polymerization rate of the crystalline propylene-based polymer (A) per gram of the catalyst per hour. Although the polymerization temperature at this stage may be identical to or different from that of the crystalline propylene-based polymer (A), it ranges from 20xc2x0 C. to 150xc2x0 C., preferably from 35xc2x0 C. to 95xc2x0 C. When the polymerization rate of the crystalline propylene-based polymer (B) is less than twice the polymerization rate of the crystalline propylene-based polymer (A) per gram of the catalyst per hour, production efficiency is sometimes reduced and the aforementioned ratio of the crystalline propylene-based polymers (A) and (B) sometimes can not be achieved.
The present invention also relates to foamed articles obtained by the aforementioned production methods. The foamed articles have good appearance and are produced using, as a foaming agent, an inorganic gas which has no environmental problems. The foamed articles of the present invention are preferably cut open by a cutting means continuously after forming to form flat sheets.